The present invention relates generally to threaded drives.
More particularly it relates to a threaded drive which includes a threaded spindle with a longitudinal axis, a supporting nut which is in screw engagement with a first predetermined clearance with the threaded spindle and is turnable relative to the threaded spindle around its longitudinal axis; and a catch nut which is in screw engagement with a second predetermined clearance with a threaded spindle, with the second predetermined clearance having a greater value than the first predetermined clearance.
Threaded drives provided with catch nuts are used in particular substantially with a vertical arrangement in the threaded spindle. The catch nut has the objective, when the supporting nut is failed, to prevent falling of the load carried by the threaded drive. This is important not only for commercial loads, but also is immensely important in particular when such threaded drives are used for lifting and lowering of operation tables and the like.
The German patent document DE 201 12 533 U1 discloses for example a threaded drive, in which the catch nut is screwed with a supporting nut so that with orderly operation of the supporting nut, the catch nut because of its greater clearance is not in screw engagement with the threaded spindle. When the screw engagement between the supporting nut and the threaded spindle fails, for example because of a breakage or the like, the catch nut comes into engagement with the threaded spindle. Because of the increased friction between the catch nut and the threaded spindle, the screw pin which holds the catch nut on the supporting nut is sheared off. As a result, the catch nut is retained in its instantaneous position and the supporting nut, because of the already destroyed screwing engagement with the threaded spindle, can freely turn on it.
In the threaded drive disclosed in the European patent document EP 0 995 714 A1, the catch nut is connected with the supporting nut by axial pins, so that the catch nut in orderly operation of the supporting nut is not in screw engagement with the threaded spindle. When the wear of the inner thread of the supporting nut increases, the inner thread of the catch nut more and more comes into screw engagement with the threaded spindle. Because of the torque produced thereby, the catch nut is gradually pulled into the supporting nut. Breaking tongs which are formed on the catch nut because of the cooperation of the wedge-shaped surfaces of the supporting nut and the catch nut are increasingly deformed toward the threaded spindle, and this increases the friction between the catch nut and the threaded spindle.
The Japanese patent document JP 8 11 082665 is also mentioned here to provide a complete picture of the prior art.
The disadvantage of the above discussed prior art solutions is that the catch nut is connected for joint rotation with the supporting nut by axial pins. When the axial openings for receiving or passage of these pins are prefabricated in the catch nut and the supporting nut, then both elements can be mounted with one another only in fully predetermined relative rotary positions. Thereby practically it is not possible to bring the catch nut in an optimal relative position with respect to the threaded spindle, or in other words a “central position”, in which the flanks of its inner thread are arranged substantially centrally between the flanks of the outer thread of the threaded spindle.
With this problem of the off-center arrangement, an extensive and complicated procedure for mounting of the catch nut on the supporting nut takes place. First the catch nut which is not provided with the axial openings must be screwed on the threaded spindle and brought in contact with the supporting nut. Then the central arrangement of the catch nut relative to the threaded spindle must be determined, and the position of the axial openings in a mounting flange of the catch nut must be indicated in correspondence with the axial openings provided in the mounting flange of the supporting nut. Then, these openings after unscrewing of the catch nut by the threaded spindle, must be drilled. Then, with a new screwing of the catch nut, it can be finally mounted on the supporting nut by the screw pins.